The present invention relates to a VoIP (Voice over IP network) gateway device and a digital one-link tandem exchange method which use a voice compression technique and packet transmission technique for a transmission line between exchanges or PBXs (Private Branch exchanges) and, more particularly, to a VoIP (Voice over IP network) gateway device and a digital one-link tandem exchange method which accommodate a plurality of IP (Internet Protocol) transit trunks in a digital exchange through a VoIP gateway that is set between the digital exchange and digital transit trunk having different transmission bit rates to perform high-efficiency coding/decoding of digital voice signals and packet transmission, thereby exchanging digital voice signals between these digital transit trunks and other digital transit trunks or intra-station extensions.
Recently, in a toll dial network using exchanges or PBXs, high-efficiency voice coding has generally been used for transmission lines. Since a deterioration in sound quality occurs upon repetition of high-efficiency voice coding and decoding, a digital one-link function is realized by some technique. The digital one-link function is a tandem exchange scheme of preventing sound quality from deterioration due to coding/decoding and an increase in circuit delay due to a processing delay by transmitting path data from a transit trunk exchange using tandem connection as coded data.
In recent years, with an increase in IP traffic typified by the explosive growth of the Internet, data and voice traffics have generally been integrated by the IP protocol. That is, the application of a VoIP gateway device to a PBX transmission line has become popular.
A tandem exchange system using a combination of a high-efficiency voice coder and TDM (Time Division Multiplex) for a transmission line has been proposed in Japanese Patent Laid-Open No. 10-336332. In this system, two high-efficiency coders which are accommodated in an exchange and oppose each other through a transit path insert predetermined sync signals in signals to be output to the respective destinations through the exchange. On the receiving side, upon detection of a sync signal, the coding and decoding sections are bypassed to prevent repetition of coding/decoding. If this technique is applied to a tandem system for performing packet transmission using an IP transit trunk, the problem of a delay due to buffering performed in a jitter buffer cannot be solved.
A conventional toll dial network in which a VoIP gateway device is applied to a transmission line has implemented the digital one-link function by IP routing using the exchange function of the VoIP gateway device.
An in-house telephone exchange system using a PBX implements various kinds of additional functions such as a PHS (Personal Handy-phone System) roaming function which the PBX has and a charging function. To activate these functions, the transit trunks in the PBX must be connected to each other in a one-to-one correspondence. This system corresponds to a conventional system using a TDM. In a toll dial network using a VoIP gateway device for a transmission line, in order to realize the above connection, a digital one-link function must be implemented independently of the exchange function (IP routing) of the VoIP gateway device.
The toll dial network using the VoIP gateway device for a transmission line, however, implements the digital one-link function by IP routing using the exchange function of the VoIP gateway device. For this reason, transit trunks in the PBX cannot be connected to each other in a one-to-one correspondence, and hence the additional functions of the PBX cannot be used.